In some spatial-division multiple-access (SDMA) communications systems, the SDMA uplinks may be assumed to be synchronous, such that symbols arriving from different data streams align according to timing boundaries. However, the potential 4 microsecond symbol timing error permitted by some Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards is longer than the 0.8 microsecond guard interval specified in the 802.11a standard. Therefore, SDMA uplink communications conducted under these conditions may actually be asynchronous, and, not synchronous as assumed. Similar problems may occur when SDMA radios conforming to IEEE 802.16 standards are used.
For more information regarding various IEEE 802.11 standards, please refer to “IEEE Standards for Information Technology—Telecommunications and Information Exchange between Systems—Local and Metropolitan Area Network—Specific Requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY), ISO/IEC 8802-11: 1999” and related amendments. For more information regarding IEEE 802.16 standards, please refer to “IEEE Standard for Local and Metropolitan Area Networks—Part 16: Air Interface for Fixed Broadband Wireless Access Systems, IEEE 802.16-2001”, as well as related amendments and standards, including “Medium Access Control Modifications and Additional Physical Layer Specifications for 2-11 GHz, IEEE 802.16a-2003”.